Natural fibre composites can have varying combinations of physical and mechanical properties, such as low density, low cost, high stiffness and strength. Kenaf (Hibiscus cannabinus) was used in this study as a natural fibre to reinforce polypropylene (PP) in the fabrication of polymer composite materials. The injection moulding method, with an injection temperature of 170°C, was used in this study. This study aimed to investigate the effect of sodium hydroxide (NaOH) on the mechanical properties of kenaf/PP using the injection moulding method. PP was mixed with different compositions (5, 10, and 15 wt%) of kenaf particles with increasing concentrations of NaOH as a treatment agent to enhance the adhesion between kenaf and PP. Morphological and structural changes of the sample fracture were observed under a scanning electron microscope (SEM). The results showed that the mechanical properties of the composite were increased when the percentage of kenaf composition was increased, and decreased when NaOH concentration was increased. The highest tensile value of the sample was 21.93 MPa at 15 wt% composition of kenaf particles, while the lowest value of 16.42 MPa was observed when NaOH was present. The improvement of flexural strength was highlighted, in 5 wt% composition of kenafreinforced PP with NaOH that was 32.07 MPa, but when the NaOH concentration was increased to over 10%, the Flexural value decreased to 26.97 Mpa. Based on the results, the researchers concluded that NaOH treatment may increase the bond strength of kenaf composite; however, increasing the NaOH concentration can lead to a decrease in mechanical properties.